These changes activate only on small screens, where "small" is defined
as <=760px, i.e. anything smaller than iPad Mini.
* Turn the sidebar into a popup (on the right) and add a hamburger
button in the header to show it.
* Split the header and footer into two lines, make the header sticky.
* Remove the prev/next links from the header.
* Put the search box first in the sidebar.
* Disable horizontal scroll of the page, enable individual scrolling
for h1, table. (Code blocks already have it).
* Allow line breaking of overly long words in the document body.
* Adjust various margins.
The sidebar popup works without JavaScript (using the "checkbox hack"),
but a bit of JavaScript is added to enable automatic closing on Esc,
on a link click and on document navigation.
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